1. Field of the Invention
The invention relates the fabrication of liquid crystal display light modulation systems on a semiconductor substrate by means of a wafer-scale process.
2. Background Information
Conventional flat-panel displays use electroluminescent materials or liquid crystals in conjunction with incident light to produce high quality images in products such as digital wristwatches, calculators, panel meters, thermometers, and industrial products. Liquid crystals are a state of matter that mixes the droplet or pouring property of a liquid and the long-range order property of a solid. This combination allows an optical activity having a magnitude without parallel in either solids or liquids. Further, when a magnetic or electrical field is applied normal to the liquid crystal material, the liquid crystal material forms a localized monocrystal that is polar in character. This localized polarization of the liquid crystal material affects the travel path of light incident to the liquid crystal material. By controlling the electrical field applied across the liquid crystal material, the travel path of light incident to the liquid crystal material can be controlled to help produce high quality images.
Modern approaches for developing high quality liquid crystal displays (LCDs), also referred to as liquid crystal spatial light modulators (SLMs), utilize an active-matrix approach where thin-film transistors (TFTs) are operationally co-located with a matrix of LCD pixels. The active-matrix approach using TFT-compatible LCDs eliminates cross talk between pixels to allow finer gray scales. Foe example, see U.S. Pat. No. 5,767,828 entitled Method and Apparatus for Displaying Grey-Scale or Color Images from Binary Images and invented by an inventor of the below disclosed invention.
Flat-panel displays employing LCD panels generally include five different layers: A white light source, a first polarizing filter that is mounted on one side of a circuit panel on which the TFTs are assembled in arrays to form pixels, a filter plate containing at least three primary colors arranged into pixels, and a second polarizing filter. A volume between the circuit panel and the filter plate is filled with a liquid crystal material. U.S. Pat. No. 5,868,951 entitled Electro-Optical Device and Method and co-invented by an inventor of the below disclosed invention relates to flat-panel displays.
Nematic liquid crystal material is frequently used in LCDs since its properties are well understood and it is easy to align. This material will not rotate polarized light when an electric field is applied across it between the circuit panel and a ground affixed to the filter plate. The first polarizing filter generally converts the incident light into linearly polarized light. When a particular pixel of the display is turned on, the liquid crystal material rotates the polarized light being transmitted through the material. Thus, light passes through the filter plate and is detected by the second polarizing filter.
Conventional liquid crystal displays such as amorphous TFT and super-twist nematic (STN) displays employ large external drive circuitry. However, the amorphous silicon transistors of conventional liquid crystal displays lack the electron mobility and leakage current characteristics necessary for micro liquid crystal displays. Moreover, size and cost restraints for micro liquid crystal displays generally require the drive circuitry of an integrated circuit to be integrated into the display along with the pixel transistors. Because the drive circuitry must be fabricated on the display substrate, micro displays are generally limited to high quality transistor technology such as single crystal (x-Si) and polysilicon (p-Si).
Micro display technologies can roughly be divided into two types: transmissive and reflective. Transmissive micro displays include polysilicon TFT displays. Polysilicon TFT displays dominate display technology in high-end projection systems and are also used as viewfinder displays in hand-held video cameras. They are usually based on twisted nematic (TN) construction. See U.S. Pat. No. 5,327,269 entitled Fast Switching 270 Degree Twisted Nematic Liquid Crystal Device and Eyewear Incorporating the Device and invented by an inventor of the below described invention.
The aperture ratio of a transmissive micro display is obtained by dividing the transmissive area by the total pixel area. High-resolution polysilicon displays such as Super Video Graphics Array (SVGA) are limited to what is considered larger micro displays having 0.9-1.8 inch diagonal (22.9-45.7 millimeter diagonal). This is because the area required by the pixel transistors and the addressing lines reduces the aperture ratio. Aperture ratios for polysilicon displays are usually around 50%. Single crystal silicon transmissive displays are similar to polysilicon TFT displays but use a transistor lift-off process to obtain single crystal silicon transistors on a transparent substrate. These displays are often referred to as Liquid Crystal on Silicon (LCOS) displays when a liquid crystal is used as the light modulator in the display.
Reflective micro displays are usually based on single-crystal silicon integrated circuit substrates with a reflective aluminum pixel forming a pixel mirror. Because it is reflective, the pixel mirror can be fabricated over the pixel transistors and addressing lines. This results in an aperture ratio (reflective area/absorptive area) that is much larger than polysilicon displays. Aperture ratios for reflective displays can be greater than 90%. Because of the large aperture ratio and the high quality silicon transistors, the resolution of a reflective micro display can be very high within a viewing area that is quite small.
There are several different liquid crystal technologies currently used in reflective micro displays. These include ferroelectric liquid crystal (FLC), polymer disbursed liquid crystal (PDLC), and nematic liquid crystal. Size and resolution of reflective micro displays may range from 0.25 inch diagonal (QVGA) to 0.9 inch diagonal (SXGA) (6.4-22.9 millimeter diagonal). Reflective micro displays are limited in physical size because as the size increases the cost increases and yield decreases.
Another aspect of liquid crystal display technology is the methodology used in their fabrication. One liquid crystal display invention has as an element an optically transmissive first substrate that may be positioned to receive light incident from the light source. The fabrication process of this particular invention teaches positioning a reflective second substrate adjacent to the first substrate. The second substrate has as active area that may include a circuit panel and a perimeter seal area surrounding that active area. To separate the first substrate from the second substrate, spacers are configured about the perimeter seal area of the second substrate. Between the first substrate and the second substrate is a liquid crystal material. In practice, the process results in individual integrated circuits, which are then laminated with a retarder and covered with a protective glass sheet. The problem with this method is that the retarder often gets scratched with a production unit so small.
For further background in this area, see Douglas J. McKnight, et al., 256xc3x97256 Liquid-Crystal-on-Silicon Spatial Light Modulator, 33 Applied Optics No. 14 at 2775-2784 (May 10, 1994); and Douglas J. McKnight et al., Development of a Spatial Light Modulator: A Randomly Addressed Liquid-Crystal-Over-Nmos Array, 28 Applied Optics No. 22 (November 1989).
The invention is a method for fabricating a light modulation system on a semiconductor substrate containing a plurality of integrated circuits. The invention is to apply the optical element layer (or a plurality of layers) at the wafer sandwich level, unlike what has been done previously, applying the optical element layer onto the individual integrated circuits. The invention takes the silicon wafer sandwich and cuts partway through the glass side. The glass layer has a transparent electrode in it. Once back in the clean room, the surface is cleaned and the retarder film is laminated on. The retarder has an adhesive (PSA) only on one side. The PSA adhesive is also laminated onto a separate piece of thin glass. The retarder with the PSA layer is laminated onto the sandwich. The thin glass with the PSA layer is brought together adhering the glass on top of the retarder. The top glass is cut through, just barely tickling the retarder. The blades are changed and the retarder is cut through. The assembly is then turned over after cutting reference flats after a correct alignment and the silicon is partway cut through. The displays are snapped apart, yielding individual integrated circuits that already have the optical element (retarder) laminated onto them, and are capable of creating separate display devices.
An alternate embodiment of the invention teaches a method by which a prefabricated top glass/PSA/retarder/PSA assembly is laminated onto the silicon wafer sandwich, thereby eliminating the operations involved in the preparation of the retarder and glass surface.
Although the present invention is described in terms of a preferred embodiment, it may be used in the fabrication of transmissive displays, reflective display systems, and emissive displays.